1. Field of the Invention
The present invention relates to gas turbine engines, and more particularly to a passive cooling system for a rim or rotor cavity.
2. Description of the Related Art including information disclosed under 37 CFR 1.97 and 1.98
As gas turbine engine evolution reaches a mature state, factors which plays an important role in deciding which engine to choose for a particular application may ultimately come down to durability and performance in the form of specific fuel consumption (SFC) or cycle efficiency and heat rate. Engine component material selection and their material properties play a major role in performance. Engine components must be cooled to acceptable levels and there are usually many trade studies performed to optimized concepts to best maintain component temperature while using the minimum amount of cooling flow. The cooling flow is supplied by compressor extraction or bleed and while work was required to compress this air, since it is used for parasitic flow purposes, it is not passed through the turbine for work to extraction. For this reason, secondary flows are usually called chargeable flows. Most conventional turbine rotor systems encompass bladed disks as opposed to blisks which are integrally bladed disks. For these conventional disks, their maximum operating temperatures are usually well below that of flow path material such as rotating blades and stationary vanes. A secondary flow designer computes the minimum rim cavity purge flows based on a particular rim seal design and as well as flow path pressure asymmetries which are a mechanism for ingestion. It is desirable to provide the minimum flow required to resolve cavity windage rise, and prevents hot gas ingestion. For robust designs, the design point for sizing cooling systems is usually at some level of engine deterioration where components, especially seals wear and the cycle is less efficient which ultimately leads to higher turbine flow path temperatures. The secondary flow designer can predict system performance at the deteriorated condition, where the life of gas path components such as blades and vanes required replacement, or in “as shipped” configuration, when the engine is shipped out of the manufacturing facility. The ideal system would be to have constant secondary flows throughout component life cycle.
A Prior Art rotor or rim cavity purge arrangement is disclosed in U.S. Pat. No. 5,181,728 issued to Stec on Jan. 26, 1993 and entitled TRENCHED BRUSH SEAL (the entire disclosure of which is incorporated herein by reference) in which a rotor cavity 82 is purged with seal leakage air flow that passes through a brush seal 70 to prevent the ingress of hot gases into the cavity which otherwise would cause a detrimental increase in the temperature and consequent reduction in life of the rotor 60. One problem with the Prior Art is that, when the brush seal wears, more seal leakage airflow passes into the cavity than when the engine was in the “as shipped” factory condition. Thus, more airflow is heated than needed, and therefore the overall engine efficiency decreases.
The concept for a constant flowing rim cavity encompasses the use of a brush seal, but can be used in conjunction with any seal that has considerable wear over time from its shipped state to the end of a components life. While brush seals have many advantageous applications in a gas turbine, one area of challenging application is in rotor rim cavities, especially stage 1 rim cavity since gas path conditions are most severe as well as flow path pressure asymmetry due to wakes from vane trailing edges. The excellent sealing characteristic that makes the brush seal a good candidate for many applications is the same characteristic that makes the application in rim seals a challenge. Since brush seals usually flow much less than the minimum required cavity flow to prevent hot gas ingestion and resolve cavity windage, when in the line to line or radially contacting position, a seal bypass hole can be provided to supply the additional amount of desired flow. U.S. Pat. No. 5,522,698 issued to Butler et al. on Jun. 4, 1996 entitled BRUSH SEAL SUPPORT AND VANE ASSEMBLY WINDAGE COVER (the entire disclosure of which is incorporated herein by reference) shows a bypass hole in parallel with the brush seal to provide purge airflow for the rim cavity. This flow is usually supplied with some injection angle in the direction of rotation to reduce rim cavity windage. The difficulty with this system is when the engine is shipped, the brush seal is not worn and the desired flows are mainly provided by the bypass holes. The problem becomes clear after brush seals wear. As the seals wear due to the cycling of the engine which lead to seal rubs, the brush seal flow consumption increases, but the bypass hole still exists and by the end of component life cycle, the system will flow as much or more that with a convection labyrinth seal. The economic advantage of the above mentioned system over conventional labyrinth seals is that the wear is exponential in nature where the labyrinth seal rub is instant. The area under the curve of flow versus time will translate to fuel savings over time. This savings is usually not enough to justify the cost of using the brush seal. A system that provides for constant flow would have the most economic advantage. FIG. 1 depicts a Prior Art device in which a brush seal 34 leakage air flow is in parallel with a bypass flow thorough passages 30.